Motor drives focus on energy efficiency

By Chris Edwards

Published Monday, March 10, 2014

Motors waste a lot of energy. Drive makers are helping them to cut down.

If you had to pick one technology to target in the bid to reduce global energy consumption, the motor would have to be one of the top candidates. The International Energy Agency (IEA) estimated in 2011 that close to half of global electricity production was consumed by electric motor-driven systems, drawing more than 7,000TWh a year and rising steadily. If not curtailed, the trend points to consumption of more than 13,000TWh per year by 2030.

Although most motors are designed for home and commercial use, the bulk of the energy goes into industrial processes - about two-thirds of the electricity usage by motors is in the industrial sector. Transport represents just 3 per cent of the total, although this figure could rise dramatically over the next decade and a half as hybrid and electric drivetrains take hold.

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Although other electrically powered systems are gaining ground - data centres now account for 10 per cent of global electricity consumption - the dominant role played by the motor in energy consumption makes it a prime target for reductions. And there is plenty of scope to cut the energy used to move things around, particularly when it comes to the relatively dumb motors used in existing water pumps, heating systems and air conditioners as well as in industry.

The waste has come to the attention of the people running the motors. According to the IEA, 95 per cent of the life-cycle cost of a motor is in the energy it uses and, as a result, the purchase price of the motor fades into the background.

John Inskip, product specialist at Siemens Drives Technologies, says: "Industry as a whole is looking at energy efficiency, looking at how it can provide the same level of service with a reduced energy intake. Whether it's airports, oil and gas, automotive or process control, they are all looking at ways to reduce energy usage."

When running at or near full speed, the difference in efficiency between the best-performing and worst motors on the market is only a few percentage points. Over the motor's lifetime this difference in itself can accumulate a substantial saving but the real savings in improving motor efficiency come from not having to run it at full pelt when not necessary.

Unfortunately, there are a lot of systems where the motor is designed to run flat out. Most of these use a separate mechanical control to divert some of the force away from the target system to provide a degree of control. Take a fluid pump as an example.

"There are a lot of applications where you only intermittently require flow in the pump itself," says Mike Lomax, sales manager at Bosch Rexroth.

A valve will reduce the fluid flow but still result in a lot of energy being wasted as the motor is continuously running at a speed intended to deal with the pipe's maximum capacity. Controlling the motor with a variable-speed drive makes it possible to remove the valve completely if the motor can halt and not just run very slowly.

Stuart Greenwood, product marketing manager at Eaton, says: "Recent research by the Carbon Trust indicates that there is significant scope for more widespread use of variable-speed technology and that UK industry could make significant savings of over £630m every year simply by using more efficient electric motors and variable speed drives. Incorporating variable-speed drives into fan, pump, compressor and cooling applications can reduce energy use by as much as 50 per cent."

Permanent-magnet motors

It's at very low speeds that the differences in motors and drives become more readily apparent. In injection-moulding, for example, there are long periods after the fluid has been pumped into the mould where the pressure needs to be maintained but there is no need for the motor to turn other than to simply hold a position at high torque. Changing the motor and its electric drive can make a big difference to energy consumption as some are less efficient than others at maintaining torque at a full stop.

Lomax points out that many machine builders prefer so-called 'squirrel cage' AC motors to the newer permanent-magnet motors because their upfront costs are much lower. However, permanent-magnet designs can provide much better overall energy efficiency for systems where the motor can be run down to a halt.

Gareth Jones, marketing director for manufacturing automation at Emerson Control Techniques, says that in general permanent magnet motors work out more efficient than AC motors for a given output: "You can reduce losses by up to 50 per cent, particularly when operating at low speeds. But we are seeing energy savings all the way up."

Lomax explains: "For a squirrel-cage motor you have to provide magnetisation current, which isn't the case for the permanent-magnet designs. AC motors don't like running down at very low speeds, whereas a permanent-magnet motor will."

Interest in permanent-magnet motors is not universal. Machine builders still tend to favour lower-cost AC motors as they are judged more on equipment price and speed than lifetime costs. But end-users in process industries have become much more sensitive to running costs.

"Within process control, there tend to be higher-power motors running more of the time, so there is more scope to save energy. People who run the plants understand the benefit of doing that," Jones says.

Lomax adds: "For end users, it's the energy cost that's driving it. Take something like a baby-feeding spoon, where it's included in every single tin of the milk, they are given out by the billion. You have to keep the unit cost as low as possible but the energy cost of producing it has been creeping up and up."

Although process control is important, Jones says the most rapid pace of adoption seems to have been in building design, helped by the adoption of efficiency standards such as VDI 4707. A high proportion of elevator contracts have gone to systems based on advanced drives and permanent-magnet motors. "There are some benefits to using that kind of technology," says Jones. "You can put it in the top of the elevator shaft, which means you don't need an engine room. So you save a lot of space as well as energy.

"The old-fashioned method was to use an AC motor and a gearbox. With a permanent-magnet motor you can remove the gearbox, which is a major source of losses, and use direct drive."

Cooling at the motor

Funding the conversion is an issue, says Abhinav Nagial, research analyst at Frost & Sullivan: "The trend varies from sector to sector. The lack of credit availability in sectors such as pulp and paper industry restricts investment in efficient drives. At the same time process industries like oil and gas, food and beverages are employing variable-speed drives to reduce the production costs."

The better efficiency of a variable-speed drive and a permanent-magnet motor can manifest in subtle ways. Lomax says: "On a lot of systems, the motor is putting heat into the fluid and then you have to have cooling as well. But if you are more careful putting heat in you don't have to take it out again."

He cites the example of presses used to form the flooring of vehicles, which needs very high pressures. Traditional designs run a pump at a constant speed to build up the pressure before a valve is opened. The running motor generates a lot of heat that demands large amounts of water cooling.

"Using a permanent-magnet motor you can bring on the pressure extremely rapidly. You can go from nought to 1,500rpm in a millisecond. With that, there is no waste at all during the idle times," says Lomax.

There is still a place for more traditional motors, says Inskip, as well as taking more care over specifications. The tendency to over-engineer to be on the safe side in terms of load capacity can lead to big inefficiencies.

"A tuned system or a more accurately specific system will be more efficient. If you have a flow and pump engineer who says they need a 5kW motor to do the job and the electrical engineer says 'I'd best be safe and use 7kW', by the time the motor is delivered it's an 11kW unit. The bigger motor is doing the same job as the 5kW motor but it absorbs more power," says Inskip.

Although the gearbox can be a source of losses, Inskip says there are many occasions when direct drive is impractical for space or engineering reasons. The gearbox and the coupling to the motor can form part of the original control strategy used by the drive. He says: "By tuning the drive and getting more stiffness into the mechanical system your responsiveness will tend to improve," he says. "You need a holistic approach."